shutdown_command
(string) - The command to use to gracefully shut down the machine once all
provisioning is complete. By default this is an empty string, which
tells Packer to just forcefully shut down the machine. This setting can
be safely omitted if for example, a shutdown command to gracefully halt
the machine is configured inside a provisioning script. If one or more
scripts require a reboot it is suggested to leave this blank (since
reboots may fail) and instead specify the final shutdown command in your
last script.